Amplifier circuit



July 16, 1940- E. B. FERRELL 2,207,975

y AMPLIFIER CIRCUIT Filed Aug.' e.' 1937 s sheets-sheer 2 FIG. .5

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Ayn A www ATTORNEY July 16, 1940 E. B. FERRELL 2,207,976

AMPLIFIER CIRCUIT Filed Aug. e, 1951 s sheets-sheet s MAM 'VVV

/Nl/ENTOR BFERRELL f4 T TOW/VE V Patented July is, 1940;

- UNITED 1 'STATES "OFl'fiCE' j AMPLIFIER CIRCUIT v v Enoch B. Ferrell, Oakhurst, J., assigner to Bell Telephone Laboratories,

Incorporated, New,A

York, N. Y., a corporationof New Ysork, l u Application `August 6, 1937, Serial `l\t r.i`157,693

5 claims. (oil-ivef'lvi) l Y able external impedances and the tube 2 is siin` This invention relates to amplifier `circuits and more particularly to circuits making use of vacuum tube amplifiers for which the basic supply of power is an alternating current source. Still more particularly-it relates to an amplifier cir-v cuit in which alternating current Voltage is supplied directly to the plate circuits of tubes without the intermediate step oi' rectification.

, In some cases I arrange the circuits'so that alof power to the tube gives substantially the same effect as is accomplished in a conventional direct current amplifier by a plurality `ofsepai'ate batteries or by batteries ofv opposed polarity.v

The invention will be better understood by reference to the following specification and the feedback connections;

accompanying drawings, in whichf Fig. 1 represents the basic principles of my circuit with the tubes connected inthe alternate arrangement;

Figs. 2 to 5 are various`.modiiications of the circuit of Fig. l, all embodying feedback circuit connections in a manner to be hereinafter described;

Fig. 6 shows an arrangement in which kthe anodes of the tubes are connected tothe same modiiied form oi feedback connection; and

Figs. l0 and 11 show certain applications-of the previous circuits to control `rpurposes. VReferring moreparticularly to Fig. 1, there is shown a circuit ofthree amplifier rtubes I,`2 and 3 connected incascade or tandem formation yA power line Illy-I I 'supplied from an. alternating current source I3 is provided withconnections f from the cathodes of the oddnumbered tubes to one side l0 ofthe alternating cu'rr'ent'power line, the alternate tube 2 havingfits cathode connected to the other` fsidey INI of the power line.

The anodes ofthe tubes I'and 3 areconnected ilarly connectedy to the side I0 of the power line. The output circuit of all but the last tube is here shownvas consistingof a resistance element I5 in parallel witha condenser IB and this external impedance constitutes also theinterstage coupling `from one tube to the next tube. 'I'he output of thelast tub'eis'shown as aicondenser I8connected to any desirable load-circuit I9. a On those Vhalf-cycles which carrythe conductor I I' positive, the first tube conducts and as a rectier builds upa voltage across its external plate resistance I5. The impedance of this tube as a rectifier, and hence the Voltage across its load resistance I5, depends upon the potential of itsgrid and the more positive is the grid with respect to 'its cathode, the greater is the voltage across 'the load resistance. The potential ofthe grid is controlled by any suitable signal impressed on the input -circuit 2|.

It will be noted that the voltage across the resistance I5 of the rst tube is negative with respect to the conductor II. Hence, according to the connections shown, the grid of the second. tube is biased negative with respect to its own cathode and this negative bias is made more negative when the first grid ismade more positive v with respect.y to the iirst cathode. The time constant ofthe impedance I5-I6 is made'suflicient- 1y high to permit the signal output from the tube I to be impressed on the succeeding tube 2 at a time when the latter has positivepotential on its anode. f

When the conductor III takes on a positive value, the tube Z-begins to conduct and the current which flows is determined in part by the potential'of its grid, which in turn iscontrolled by the potential difference across the interstage coupling betweentubes vvIv and 2 and thispotential in turn is` controlled by the signalvoltage at the l In precisely the same way the tube 3 is rendered conducting atthe same time as the tub-e I, during which time the v.tube 2 is not conducting. The current, however, which flowed through the tube 2 has vpermitted the building up of a charge in the interstage couplingbetween 2 and 3, which charge maintains a potential on .the grid of tube 3 in'spite of the fact ,thatthe tube 2 has become non-conductive. n Obviously, the number of tubes which are connected in cascade formation may be increased to such extent as is desired and in the output of the last tube there is connected a maintained bythe .Condenser '.82

It will be noted that the instantaneous value of the potential of the anode of such a tube as i tube I is given by the expression where E is the peak value of the voltage across the power line,

ep is the numerical value of the direct current voltage across the plate resistance I5, and

ec is the numerical value of the negative direct current voltage supplied to the grid.

It is rapparent that the tube I cannot conduct one cycle to the next of the alternating current supply. This means that the amplier will vnot follow variations-of the input voltage at a Vrate 'ashigh as that of the alternating current source and the circuit is therefore more peculiarly adapted for amplifying signal frequencies of lower value than the frequency of the alternating current source. Thus, if the source should be of the usual 60-cycle frequency, the circuit is especially adapted for what is commonly called a direct current amplier or for the amplification of Signals, theessential frequencies of which are lower than the (l0-cycle frequency of thepower line. If it is desired to amplify a band of frequencies extending from 0 to f, inclusive, a source of lsupply should be used having a frequency higher than f. v

In general, undesirable modulation products involving combinations of the power frequency and the signal frequency will be formed. Some of these will be of frequency lower than the power supply. Hence, it may be desirable and in some cases necessary to space the power frequency well above the signal frequency. For example, if it is desired to amplify direct current and all frequencies up to l0 kilocycles, one may use a power supply at 50 or 100` kilocyc'les. In this case also it will be desirable that the interstage coupling circuits shall take on denite filter characteristics, which for the example cited might lbe arranged to cut off at 12 or 15 kilocycles, suppressing frequencies above the cut-off value. Such lters may be used to replace any or all of the interstagecouplings and may, if desired, also be used in the outputv circuit.

It will be recognized that this amplifier inthe simple form of Fig. 1 would be likely to give rise to considerable distortion and be subject to the objections of instability with supply of voltage variations, tube aging, etc. For this reason I nd it advantageous to supply a negative feedback which has the property of increasing stability and decreasing distortion occurring in the circuit. Such a feedback connection may be introduced in a variety of ways. Thus, referring to Fig. 2, it will be'noted that there are two tubes I and 2 connected in reverse manner to the main supply, having an interstage coupling and an output circuit similar to that. of Fig. `1. Across the output circuit of the tube 2, however, is shown the resistance 23, serving as a potentiometer, from one point of which there is aconnection `to the cathode of tube I. The feedback connection comprises, then, a portion of the resistance 23 and also the condenser 24, the two elements constituting a circuit with a time constant rcom'parableto that of the circuit I5v-II.

vIf in the circuit' of Fig'. 2 th rst grid is driven 'ance' 25 in series with the cathode.

positive, therst tube draws more current. The

first plate, therefore, carries the second grid more negative with respect to the second cathode and the second tube then draws less current, andthe second plate now becomes less negative with respect to its. power line. This is a movement in the positive direction and if a part of this is applied to the first cathode as shown, it decreases i the effect of the initially assumed positive drive on the rst grid. The feedback is, thereforaa negative feedback. f f

Fig. 3 shows asimilar arrangement using three stages. It is also possible to use four stages with the feedback voltage taken from the plate of the fourth tube inthe manner shown in Fig. 2.

It will be noted that the feedback of Fig. 2 is a voltage feedback, but this feedback voltage may be developed across a small resistance in series srable'in that it may take a voltage which is'25 which may begrounded, for example,l and def negative lfrom one of the power supply wires,

livers finally avvoltage which is negative' from the same reference point. In this way it would be observed that one may have a circuit with an. v

output which has one side common-'tothe input.vv Also, this feature of an output with one sidevv 'common to the input may be obtained with greater amplification by' the arrangementv of Fig. 4. Here the rst tube I has a large resist- Itis, therefore, highly degenerative andhasa lo'w but quite constant gain. `It serves principally as a 'phase inverter, butit will be noted-'that one side `of the input circuit is common with one side of the output circuit. Also, this Fig. 4 involves two feedback amplifiers, one containing a single stage vcornprising'the tube I and the other containing three-stages comprising the tubes 2, 3 and d, the feedback in this case beingV accomplished through the resistance'v 21. Y

yAs-a variation of the feedback connection, Fig.

5 shows a total of four stages made upV olf-two' two-stage feedback amplifiers in cascade, and it becomes apparent now that a great variety of combinations of one, two, ormore stages of feedback amplifiers could be made.

While fthe invention has thus far been described as -one in which alternate cathodesr are tied to opposite sides of the 'powersupply line,

this is not necessary. Thus, Fig. 6 shows -an` l powerl line on the one side alone, forfit will Vbe understood that'in all of the circuits the current of the last tube is ordinarily much larger than thatgof thefothers. i

In this 'circuit of Fig. 6, as well .as in the others i of lthis application, there will be occasions when v the frequencies due yto the primaryv alternating current source will be troublesome.` Thesewill to a large'extent besuppressedby the circuits kI 5, I6 but in casetheyresidual variations are still troublesomethey may be suppressedby meanislof i wave filters introducedat 'one point Vor lanother ing lthefinductaneezs and-the capacityv 29. ob: viouslymore elaborate filters may be introduced at. this or. other pointsas one may desire.

rig. .711s a,.fmodincation ofthe circuit eifrig.V c,v I there being shown here three stages of amplific'ation, with' the. ,cathodes all connectedA t'o one sidevof the power line and witha-.negative feedbackintroduced bythe resistance 2'I.j `It will also beV noted in thisica's'e that `the voutputvcircuit has one side'commo'ni-.to the inputfcircuit.Y l

i' Figkf'shows yafcombinationfof these twoarrangements. "I'husg'ithe cathode ofl theiirstV tube is connected toone side of the power line and the cathodesiofallthe-other tubes are connected in parallel to the'other side of the poweniine.v

The first tube acts virtually as a phase inverter,

y in a manner analogous to that of tube I in Fig. 4.

The' interstage coupling arrangement from tube I to tube 2 is similar to that of Fig. 1, and the interstage couplings between tubes 2-3 and tubes 3-4 are thesame as that shown for Figs.

6 or 7. l Fig. 9 illustrates an extension of the arrange- I ments for providing lfeedback connection, the' one here shown being that commonly known as the bridgevtype of feedback. In Fig. 9, Ro repre vsents the internal impedance ofthe last tube'.

The resistances Rn, Ri, Rz and R3 form a balanced bridge with the points AB conjugate to the points CD. Under these circumstances, the voltage between A and B is dependent upon the apparent generator voltage inthe last tube and is fed back to the grid of the first tube, but this voltage between A and B is independent of changes occurring inthe load circuit, all in the manner which is well known in the art.

An amplier of the type described above is particularly applicable to control circuits such as thosefor automatic volume control, rectifier output regulation, battery charging, alarm signal operation, etc. f In the automatic volume control circuit as appliedto many radio receiving sets the negativev bias applied to various tubes should be proportional to, but many times as large as the y be used as a reference.

10 in which the group of amplifiers I, 2 and'3r are arranged in tandem in substantiallyr the manner indicated in Figs. 6 and 7. 'I'hetube or tubes serve as a rectifier to deliver direct current power at the terminals I9 from the alternating current power supply. Across the direct current powerv line' is bridged a volt coil 3I, and a feedback connection from a suitable point thereon goes to the grid of tube I. In this feedback grid circuit there is also included a reference voltage from a source 32. Ifv the voltage lamplification in thisI circuit after feedback is, say, 100, and if y we use as the input 'or' reference Voltage 32 a 1.5

' volt dry cell, then the output across the terminals I9 is 150 volts and injso far as the negative feedback has made the amplification ratio independent of time, load, supply voltage, etc., this output voltage wi11 a1so be independent of those variables. It will be observed that the source of reference voltage 32 need not deliver any appreciable'r current and may, therefore, consist of small cells designed for long shelf life.-l For more precise regulation some of the well-known standard voltage vcells can be used. l A 4`Still another particular illustration of the application of my" invention is shown inflig. 11 in which the upper' part' A' of the figure represents a `conventional rectifier supplying direct current powerto the terminals I9.y The rectifier tubes 30 havefgrid control. Due to variations in the load orin Vthe alternating current supply the voltage acrossthe terminals I9 will in general, be subject to nuctuations. l To ,protect against this,.there y is provided as shown in the lower part ofFig. `311 a direct current amplifier feeding part of its output back to the grids ofthe rectifier tubesv vin the portion A. Any variations in ,the voltage across the terminals vI 9 are effective, through the in the vtrain of direct currentampliiierv tubes. The particular circuit of Fig. 11 is aplicable to high vacuum recti'fying tubes, eachtube comprisingY three elements, one of which is a control grid, such as are well-known inthe art'. It is, however, evident that any type of rectifier tube may be used so ong as these rectifier tubes are volt coil 3|, to `operate von the grid of the first subject to control.y Thus, they maybe replaced by gaseous rectiers of the lhot cathode or ignitron type, if these are at the same time provided with suitable alternating current' excitation of the grids. In this Fig. 11 a separate secondary winding 34 is shown as providing power for the plate circuits of the direct current amplifier.

,'Ihis, however, is a matter `of convenience and the necessary alternating current voltage could be obtained by tapping from the main high voltage winding. A division of this application, directed particularly to the subject-matter of Figs. 10 and 11, .was filed March 13, 1940, as application Serial No. 323,689, for Amplifier circuits.

While this invention has been described in 'terms of three-electrode tubes, .this is for the sake of simplicity. It is understood that any or all of these tubes may be lreplaced b y tubes of other types such as screen gridtubes or pentodes. The substitution of these will be obvious to thosev skilled in the art.

What is claimed is: 1,. An amplifierl circuit comprising a plurality of electrondischarge devices in tandem, an alterb nating current power source connectedtothe cathodes and anodes of said devices such that alternate devices operate on alternate half cycles of said supply, a negative feedback circuit for the initial discharge device, and a negative feedback circuit for the succeeding devices, said latter feedbackcircuit including an impedance commonto the cathode-anode circuits. of each'of said succeeding devices.

2. An amplifier circuit comprising a plurality of electron discharge devices in tandem, an alterhating currentv power source connected tothe cathodes and anodes of saidy devices such .that alternate devices operate on alternate half cycles of "said supply, a negative feedback circuit for the initial discharge device, and a negative feedback circuit for the succeeding devices, said latter feedback circuit including an impedanceA common to the cathode-anode circuits of each of saidsucceeding devices, one side of the input circuit of the amplifier being common with one side of the output circuit of the amplifier..

43. An amplifier circuit comprising la plurality ofel'ectron discharge devices in tandem, an atl-175Al ing devices, said latterjfeedback circuit includternating current powersource connected to the cathodes` and anodes of said devices, a negative feedback circuit Afor the yinitial discharge device, and -a negative feedback circuit for the succeed--I ing devices, said latter feedback circuit including Yan impedance common to the cathodefanode circuits of said succeeding' devices. l n

4. A n arnplier circuit comprising a plurality of electron discharge devices in tandem, -an a1` ternating current power source connected to the' cathodes'and anodes of saiddevices, :a negative feedback circuit nfor the initial discharge device, ancla negative feedback circuit for the succeed-1 mordre ing -an impedance common 'to the cathode-anode circuitsyof f each `of said" succeeding devices, one

side of the'inputfcircuittofthe amplifier being common: with-fone, side of' the.. output circuit of the amplifier. A t 5. An .famplier circuit comprising a plurality of electrondischarge devices in tandem, ank alternating current power sourcey connected to the cathodes and anodes.- of said devices, and a neg- 1 ative feedback connection `for said devices'comprising an impedance Acommon to the cathodef anode `circuitsaof said ydevi'ces.

kENOCHv B. FERRELL.l 

